Further, Since the device came to me in the S-OFF and SuperSU installed on it (Rooted), I managed to install twrp-2.7.0.2-m8.img, for which now I have backed up my complete system, and have nandroid backup for it.

Now, I am curious to install other custom rom as well, my earlier readings / understandings here & after googleling, warns me to do it untill I have the Firm ware updated.